the missing earthquakes of humboldt county:...

S41A-1899

THE MISSING EARTHQUAKES OF HUMBOLDT COUNTY: RECONCILING RECURRENCE INTERVAL ESTIMATES, SOUTHERN CASCADIA SUBDUCTION ZONE Jason R. Patton 1, 2 and Tom H. Leroy 2, 3

1. COAS, Oregon State University, Corvallis, OR, United States. 2. Cascadia GeoSciences, Cascadia GeoSciences, McKinleyville, CA, United States. 3. Pacific Watershed Associates, McKinleyville, CA, United States

PACIFICWATERSHEDASSOCIATES

C

Elev

atio

n a

bov

e N

AV

D88

(m)

2

1

0

2.25

ESB-7 ESB-8 ESB-6 ESB-5 ESB-4 ESB-3 ESB-2 ESB-1

Eureka Slough cores (Pritchard, 2004)

2

1

-1

0

2.25

ARC-1ARC-0 ARC-3ARC-2 ARC-4

Arcata salt marsh (Pritchard, 2004)East

Elev

atio

n a

bov

e N

AV

D88

(m)

Elev

atio

n ab

ove

NA

VD

88 (m

)

2

1

0

-1

2.25

JAC-1 JAC-2 JAC-3 JAC-4 JAC-5 JAC-12 JAC-11 JAC-10 JAC-9 JAC-8JAC-7

Jacoby Creek cores (Pritchard, 2004)

JAC-6

MRS-6 MRS-C5 MRS-3 MRS-4 MRS-1 MRS-2 MRS-5 MRS-C10 MRS-C16 MRS-C20 MRS-8 MRS-7

Mad River Slough (Vick, 1988)

0

1

2

3

Dep

th b

elow

gro

und

surf

ace

(m)

Lagoon Creek (Carver and others, 1998)LCVB 18 LCVB 12 LCVB 17 LCVB 11 LCVB 16 LCVB 15 LCVB 4 LCVB 5 LCVB 6 LCVB 14 LCVB 7 LCVB 8 LCVB 9 LCVB 10 LCVB 13 LCVB 19 LCVB 20 LCVB 21 LCVB 3 LCVB 2 LCVB 1

0

1

2

3

Dep

th b

elow

gro

und

surf

ace

(m)

Crescent City marsh (Carver and others, 1998)CC-1 CCVB-14 CCVB-12 CCVB-1 CCVB-2 CCVB-3 CCD-4 CC-2 CCVB-5 CCVB-6 CC-3 CCVB-7 CCVB-8 CCVB-9

0

1

2

3

Dep

th b

elow

gro

und

surf

ace

(m)

Pay-C ES-E Pay-ER Spadons Pay-A

First Slough cores (Valentine, 1992)

0

1

2

3

4

5

6

Dep

th b

elow

gro

und

surf

ace

(m)

0

1

2

3

4

5

6

Dep

th b

elow

gro

und

surf

ace

(m)

South Bay East cores (Valentine, 1992)

Fay-E Eureka Slough 2 Eureka Slough 1

SB-ASB-E


0

1

2

3

4

5

6

Dep

th b

elow

gro

und

surf

ace

(m)

Island Hookton SloughTwo Islands

Hookton Slough


0

1

2

3

4

5

6

Dep

th b

elow

gro

und

surf

ace

(m)

0

1

2

3

Dep

th b

elow

gro

und

surf

ace

(m)

SBVB-1 SBVB-2 SBVB-3 SBVB-8 SBVB-4 SBVB-5 SBVB-9 SBVB-7 SBVB-6 SBVB-10

South Bay West cores (Carver and others, 1998)

0

1

2

3

ER-VC-11 ER-VC-10 ER-VC-05 ER-VC-07 ER-VC-08 ER-VC-09

Eel River Delta cores (Li, 1992)

Dep

th b

elow

gro

und

surf

ace

(m)

3

2

1

0

-1

-2

Elev

atio

n (m

eter

s)

Swiss Hall cores (Witter and others, 2002)LS5 LS4 LS3 LS2 LS1 LS0.5 LS0 LS-1 LS-2LS6LS7LS13 LS12 LS11 LS8

Hookton Slough (Patton, 2004)

Elev

atio

n a

bov

e N

AV

D88

(m)

1

0

-1

1.25

-1

5a 5b 49 47 29 27 26 50 48

References

Atwater, B. F., Stuiver, M., and Yamaguchi, D. K., 1991. Radiocarbon test of earthquake magnitude at the Cascadia subduction zone. Nature, v. 353, p. 156-158.

Clarke, S. H., and Carver, G. C., 1992. Late Holocene Tectonics and Paleoseis-micity, Southern Cascadia Subduction Zone, Science, vol. 255: 188-192.

Carver, G.A., Abramson, H.A., Garrison-Laney, C.E., and Leroy, T., 1998, Pale-otsunami evidence of subduction earthquakes for northern California: Final Report for Pacific Gas and Electric Co., 164 p., plus appendices.

Flück, P., Hyndman, R. D., Rogers, G. C., and Wang, K., 1997. Three-Dimensional Dislocation Model for Great Earthquakes of the Cascadia Subduction Zone, Journal of Geophysical Research, vol. 102: 20,539-20,550.

Garrison-Laney, Carolyn E. 1., AbramsonWard, Hans F.2., and Carver, Gary A., 2006, in Hemphill-Haley, M.A., McPherson, R., Patton, J.R., Stallman, J., Leroy, T., Sutherland, D., and Williams, T., eds., 2006 Pacific Cell Friends of the Pleistocene Field Trip Guidebook, The Triangle of Doom: Signatures of Quaternary Crustal Deformation in the Mendocino Deformation Zone (MDZ) Arcata, CA.

Goldfinger, C., Grijalva, K., Bürgmann, R., Morey, A., Johnson, J., Nelson, H., Gutiérrez-Pastor, J., Ericsson, A., Karabanov, E., Chaytor, J. D., Patton, J., and Grácia, E., 2008. Late Holocene Rupture of the Northern San Andreas Fault and Possible Stress Linkage to the Cascadia Subduction Zone in BSSA, v. 98, no. 2, pp. 861-889, 2008.

Leonard, L., Hyndman, R. D., and Mazzotti, S., 2004. Coseismic subsidence in teh 1700 great Cascadia earthquake: Coastal estimates versus elastic dislocation models, GSA Bull., v. 116, n. 5/6, p. 655-670.

Li, Wen-Hao, 1992, Evidence for the late Holocene coseismic subsidence in the lower Eel River Valley, Humboldt County, northern California: an appli-cation of foraminiferal zonation to indicate tectonic submergence: M.S. thesis, Arcata, California, Humboldt State University, Department of Geol-ogy, 87 p.

Nelson, A. R., Asquith, A. C., and Grant, W. C., 2004. Great Eartquakes and Tsunamis of hte Past 2000 Years at he Salmon River Estuary, Central Oregon Coast, USA, BSSA, v. 94, n. 4, 2004.

Oswald, J., Leroy, T. H., and Williams, T., B., 2009. Deofrmation Associated with the Northern Migration of the Mendocino Triple Junction: the Mendocino Deformation Zone. GSA abstracts with Programs, v. 41, n. 7, p. 480.

Patton, J. R. and Witter, R. W., 2006. Late Holocene Coseismic Subsidence and Coincident Tsunamis, Southern Cascadia subduction zone, Hookton Slough, Wigi (Humboldt Bay), California, in Hemphill-Haley, M.A., McPher-son, R., Patton, J.R., Stallman, J., Leroy, T., Sutherland, D., and Williams, T., eds., 2006 Pacific Cell Friends of the Pleistocene Field Trip Guidebook, The Triangle of Doom: Signatures of Quaternary Crustal Deformation in the Mendocino Deformation Zone (MDZ) Arcata, CA.

Pritchard, C. J., 2004. Late Holocene Relative Sea-Level Changes, Arcata Bay, California: Evaluation of Freshwater Syncline movement using coseismically buried soil Horizons. M.S. thesis, Arcata, California, Humboldt State Univer-sity, Department of Geology, 56 p.

Bronk Ramsey, C., Higham, T.F.G., Brock, F., Baker, D. & Ditchfield, P., 2009. Radiocarbon dates from the Oxford AMS system: Archaeometry Datelist 33, Archaeometry, v. 51, n. 2, p. 323-349.

Reimer, P., J., Baillie, M. G. L., Bard, E., et al., 2004. INTCAL04 Terrestrial Radio-carbon Age Calibration, 0-26 cal Kyr BP., Radiocarbon, v. 46., n. 3, p. 1029-1058.

Valentine, D. W., 1992, Late Holocene Stratigraphy, Humboldt Bay, California: Evidence for Late Holocene Paleoseismicity of the Southern Cascadia Subduction Zone: M.S. thesis, Arcata, California, Humboldt State University, 82 p.

Verdonck, D., 2006. Contemporary vertical crustal deformatino in Cascadia. Tectonophysics, v. 417, p. 221-230.

Vick, G., 1988, Late Holocene Paleoseismicity and relative vertical crustal movements, Mad River Slough, Humboldt Bay, California: M.S. thesis, Arcata, California, Humboldt State University, 88 p.

Wang, K., He, J, Dragert, H., and James, T., 2001. Three-dimensinoal viscoelas-tic interseismic deormation model for hte Cascadia subduction zone, Earth Planets Space, v. 53, p. 295-306.

Wang, K., Wells, R., Mazzotti, S., Hyndman, R. D.,, 2003. A reivsed dislocation model of interseismic deformation of the Cascadia subduction zone, J. G. R., v. 108., n. B1.

Witter, R. C., Patton, J. R., Carver, G. C., Kelsey, H. M., Garrison-Laney, C., Koehler, R. D., and Hemphill-Haley, E., 2002. Final Technical Report: Upper Plate Earthquakes on the Western Little Salmon Fault and Contemporane-ous Subsidence of Southern Humboldt Bay over the past 3,600 Years, North-western California. United States Geological Survey, National Earthquake Hazards Reduction Program, Award No. 01HQGR0215, 37 p.

Project LocationSurveyedElevationControl?

Biostratigraphy?All Events

Dated and Correlated?

14C Limitations1 DescriptiveLimitations2

ScientificValue of the Site3

Prioritization for Conducting

Additional Studies

Crescent City marsh no diatoms no N,L A 1 5

Lagoon Creek no diatoms yes I A 3 12

Mad River Slough no radiolarians yes B,L,I S 1 1

Arcata Marsh yes diatoms yes A,L A 1 2

Jacoby Creek yes no yes A,L A 1 3

Eureka Slough no no yes B,L S 2 6

First Slough no no yes B,L S 2 7

Fay Slough no no no B,L S 2 8

South Bay (west) no diatoms yes A A 3 10 The data here is mostly sufficient, occupying a new site would be more prudent.South Bay (east) occupies an area subject to repeated Late Holocene coseismic

Table 1. Data limitations and priorities for conducting additional paleoseismic and paleotsunami studies at site specific locations in Northern California.

Notes

The event horizons at the Crescent City marsh are poorly constrained. There is a high likelihood that the marsh archives teletsunami evidence.The data there is mostly sufficient and the lagoon was found to contain dangerous levels of dioxin.

Eureka Slough, First Slough and Fay slough occupy an area subject to repeated Late Holocene coseismic subsidence, the timing and magnitude of the associated land level changes is poorly understood. This area is also occupied by critical transportation infrastructure and an airport protected by tidal levees.

Mad River Slough, Arcata Marsh and Jacoby Creek occupy an area subject to repeated Late Holocene coseismic subsidence, the timing and magnitude of the associated land level changes is poorly understood. This location is also occupied by critical transportation infrastructure and a public water treatment facility vulnerable to relative sea-level changes.

South Bay (east) no no no B,L,N,D S 2 9Swiss Hall yes no yes A A 3 11Hookton Slough yes diatoms yes A A 3 13

Eel River no forminifera yes B,L,N S 1 4

The Eel river valley lies just north of the triple junction and has the potential to record paleosiesmic and paleotsunami evidence at the southern most Csz. This data would be valuable in understanding how the southern Csz transitions to the San Andreas fault zone.

South Bay (east) occupies an area subject to repeated Late Holocene coseismic subsidence, the timing and magnitude of the associated land level changes is poorly understood. This location is also occupied by critical transportation infrastructure and a community college.The data here is mostly sufficient, occupying a new site would be more prudent.The data here is mostly sufficient, occupying a new site would be more prudent.

1 14C limitations include: B-bulk samples of organic material were submitted for analysis; I- Inverse dates were encountered after analysis of the data; N- Not all of the disturbance events had 14C determinations; L- several or all event horizons have a limited number of samples to statistically verify age determinations; D-Anomalous age determination within data set; A-14C sample collection techniques and analysis meet current scientific standards.2 Descriptive limitations include: A- Lithologic descriptions of core logs meet current scientific standards; S- Lithologic descriptions of core logs are simplified compared to modern scientific standards.3 Scientific value of site is a subjective determination based on the specific attributes of the site including: ability to archive disturbance events, the value of the existing data, proximity to large human populations or valuable infrastructure, if it spatially or temporally occupies a known or important data gap, or if the data collected at the site meets current scientific standards and does not currently need further analysis. The sites are evaluated on a scale from 1-3 with 1 assigned to locations that are a top priority for further investigation and 3 being locations that do not merit further investigation at this time.

0500

1000

1500

2000

2500

3000

3500

4000

4500

5000

5500

39.540.5

41.542.5

43.544.5

45.546.5

47.548.5

49.5

Calendar years (BP 1950)

Latitude (N)

T1T2T3T4T5T6T7

T3aT5b

T4a

T2a

T5aT6a

T10 T9 T8

T10b

T10f

T9a

T8a

T7a

010002000300040005000Calibrated Age (cal yrs BP)

R_Combine COMBINE event 1


R_Date ER_1987_ER-VC-11_3_3_0



R_Date SWB_1994_SBVB4-RC5_1_1_1

R_Combine SW Bay event 2

R_Date SH_2000_00-LS-22_2_2_0

R_Combine Swiss Hall event 3


R_Date HS_2001_01br05a 47_1_1_1

R_Date HS_2001_01BR02_196B_3_3_0

R_Combine Hookton Slough event 4


R_Date HS_1990_HS1-689_1_1_0

R_Combine South Bay event 2

R_Date HS_1990_HS-B-275-A_2_4_0

R_Date SB_1990_SB-A-405-A_1_5_0


R_Date SB_1990_SB-E-625_3_7_0

R_Date SB_1990_SB-A-655_4_8_0

R_Date JC_1990_JC A_0_1_1

R_Date 0_1990_WOOD-A-70-1_0_2_0

R_Date ES_1990_ES-E-300-A_0_4_0

R_Date PAY_1990_PAY-C-118_0_5.5_0

R_Date PAY_1990_PAY-A-300_0_6_0

R_Date PAY_1990_PAY-A-365-1_0_7_0

R_Date PAY_1990_PAY-A-430-1_0_8_0

R_Date ES_1990_ES-E-645_0_9_0

R_Date ES_1990_ES-E-695_0_10_0

R_Date AS_2003_ARC-4-102_1_1_1

R_Date MRS_2003_MRS-3-287_3_3_0

R_Combine Pritchard event 4

R_Date MRS_1987_MRS-3-BP280_4_4_0

R_Date MRS_1987_MRS-3-3689-1_5_5_0

R_Combine North Spit event 1

R_Date OK_1996_unk_1_1_1

R_Date LC_1992_LC1-B6_2_2_0

R_Combine Lagoon Creek event 4

R_Date LC_1992_LC-4-RC2_5_5_0

R_Date LC_1992_LC-4-RC1_6_6_0

R_Date LC_1992_LC-16-RC5_7_7_0

R_Date LC_1992_LC-16-RC6_8_8_0

R_Date CCS_1996_CC 18/61.5_1_1_0

R_Date CCS_1996_CCVB12-RC3_2_2_0

R_Date CCS_1996_CC 41-180_4_4_0



R_Date CCS_1996_CC 18/61.5_1_1_0


R_Date CCS_1996_CC 41-180_4_4_0


R Date CCS 1996 CCVB14-RC1 6 6 0_ _ _ _ _ _

R_Date LC_1992_LC1-B6_2_2_0

R_Combine Lagoon Creek event 4

R_Date LC_1992_LC-4-RC2_5_5_0

R_Date LC_1992_LC-4-RC1_6_6_0

R_Date LC_1992_LC-16-RC5_7_7_0

R Date LC 1992 LC-16-RC6 8 8 0_ _ _ _ _ _

R_Date OK_1996_unk_1_1_1

R_Combine North Spit event 1_ p

R_Date JC_1990_JC A_0_1_1

R_Date 0_1990_WOOD-A-70-1_0_2_0

R_Date ES_1990_ES-E-300-A_0_4_0

R_Date PAY_1990_PAY-C-118_0_5.5_0

R_Date PAY_1990_PAY-A-300_0_6_0

R_Date PAY_1990_PAY-A-365-1_0_7_0

R_Date PAY_1990_PAY-A-430-1_0_8_0

R_Date ES_1990_ES-E-645_0_9_0

R_Date ES_1990_ES-E-695_0_10_0

R_Date AS_2003_ARC-4-102_1_1_1

R_Date MRS_2003_MRS-3-287_3_3_0

R_Combine Pritchard event 4

R_Date MRS_1987_MRS-3-BP280_4_4_0

R_Date MRS_1987_MRS-3-3689-1_5_5_0

R_Date HS_1990_HS1-689_1_1_0


R_Date HS_1990_HS-B-275-A_2_4_0

R_Date SB_1990_SB-A-405-A_1_5_0


R_Date SB_1990_SB-E-625_3_7_0

R_Date SB_1990_SB-A-655_4_8_0_ _ _ _ _ _

R_Date HS_2001_01br05a 47_1_1_1

R_Date HS_2001_01BR02_196B_3_3_0


R Combine Hookton Slough event 5_ g

R_Date SH_2000_00-LS-22_2_2_0


R Combine Swiss Hall event 4R_Combine Swiss Hall event 4

R_Date SWB_1994_SBVB4-RC5_1_1_1

R_Combine SW Bay event 2_ y



R_Date ER_1987_ER-VC-11_3_3_0


R Combine COMBINE event 5R_Combine COMBINE event 5

Gold

finger, et al., 2008

Eel Canyon

Trinidad Canyon

Smith Canyon

Rogue Canyon

JdF

Cascadia Channel

Astoria Channel

Hydrate Ridge

Barclay Canyon

Crescent City

Lagoon Creek

Orekw’

North SpitArcata Bay

South Bay

Southwest Bay

Swiss Hall

Hookton Slough

Eel River

126°W

125°W

125°W

124°W

124°W

123°W

123°W

40°N40°N

41°N41°N

0 5025Kilometers

P

VM

Co

E

KSFG

ED

SN-GV

Cn

F

Cn

SCP

V

M

K

SN-GV

G

Co

E

ED

F

Gorda - Block comprising the subducting Gorda Plate at southern end of the Cascadia subduction zone. This block is internally deforming in response to 1) subduction 2) northward impingement of the Pacific plate.

South Cascadia - Defines northern end of study area. Block characterized by the accretionary prism with deformation consistent with subduction and not directly influenced by migration of the Pacific - North America boundary.

Pacific- Block comprising the Pacific Plate bound on the north by the Mendocino transform and on the east by the San Andreas fault transform margin.

Vizcaino – Microplate of the northeastern Pacific Plate attached ~11 my (Godfrey, 1998). Its boundaries may influence the location of the northern San Andreas transform margin and the northeast-vergent uplift related to formation of the King Range topographic high.

Klamath - Defined by the post-Cretaceous terranes of the Klamath Mountains Province. Considered a semi-rigid block and moving northwest towards the accretionary margin of southern Cascadia and rotating clockwise. Buttresses subduction zone accretionary structures on the west, and buttresses Eastern block in the south.

Sierra Nevada-Great Valley – Inland eastern edge of study area defined by rigid northwest motion that converges with Eastern and Klamath blocks.

Coastal – Fault bounded block between the San Andreas and Maacama fault zones that defines the western extent of the transform margin between Pacific and North American Plates. Moves north at about 3.5 cm/yr.

Central - Fault bounded block between the Maacama and Bartlett Springs-Lake Mtn fault zones within the central portion of the transform margin between Pacific and North American Plates. Moves north at about 2.5 cm/yr.

Eastern - Block east of the Bartlett Springs-Lake Mtn fault zone and defined on the east by the contact of Franciscan ultramafics and the Great Valley sequence. Northern area is characterized by regional uplift as a result of impingement with Klamath Block to north. Moves north-northwest at about 1.5 cm/yr.

Mendocino – Region above the slab window that transitions from predominately strike-slip to predominately compressional deformation. Defined on the north by upper plate reverse faults (Russ fault), on the south by the isostatic gravity anomaly low, and in the east by the Bartlett Springs-Lake Mtn fault zone. Through going dextral slip faults begin to right-step and strike more northerly across this block.

Eel Delta – Block contains the Eel River basin and is bound on the south by the Russ fault, and on the north by the Little Salmon fault zone. North-South contraction is accomodated across the Eel River syncline and is being impinged upon from the south. Block moves ~2 cm/yr northward as transition to subduction zone begins.

Freshwater – Block contains the upper plate thrust faults and folds within the southern Cascadia subduction zone. Bound on the south by the Little Salmon fault zone, on the northeast by the Big Lagoon-Bald Mtn fault zone, and on the east by the Eaton-Roughs-Lake Mtn fault zone. Eastern boundary accommodates inland strike-slip deformation east of the accretionary prism and north of Cape Mendocino.

South Fork – Block west of the Klamath Block and immediately east of the upper plate thrusts faults coupled with the subducting plate. The southern boundary is near the southern extent of the (E-W) subducted slab. Eastern extent of dextral deformation; does not propagate east of the Klamath block.

SF

Oswald, et al., 2009

Regional Tectonic Regimes (Oswald and Gullick)

50 0 50 100 150 Kilometers

?

SW

FT

KA

TRUR

GP

PP

US

SR

?

123°40°

42°

126°

Klamath ArcForearc TtranslationUndeformed SynclineSubduction RegimeTranspressive RegimeUplift Regime

Pacific PlateGorda PlateCA Coastline500m ContoursS. Edge of Gorda

Slab Window

N. American Plate

Gulick and Meltzer, 2002

Verdonck, 2007

Uplift profile S to N, grey = calculated, black = model

Leonard, et al., 2004

Mean subsidence estimates a) after 800 and 550 yrs of convergence and b) uniform slip of 10, 20, 30 and 50m.

Wang, et al., 2003

CASC3D-2 model: a) strain rates, b) model and GPS velocities, and c) model and tide gage uplift rates.

Mean subsidence rates calculated at sites comparred with elastic dislocation

model estimated subsidence.

Leonard, et al., 2004

GORDA PLATE

JUAN DE FUCA PLATE

Wang, et al., 2001

Fluck, et al., 1997

Uplift rates at 300 yr. after the great earthquake

Locked and transition zones of Cascadia Megathrust (based on thermal constraints and geodetic data) used in elastic dislocation model.

Thrust fault atplate boundary

Other faultsSpreading ridge200-m isobathDeep-sea channelVolcanoEarthquake evidenceTsunami deposit

0 100 200

km

300

130 o

130 o

125 o 120 o

50o

45o

40o

Queen Charlottefault

125 o120 o

50o

45o

40o

Continental shelf

WillapaBay

Coos Bay

Coquille RiverSixes River

PacificPlate

Paci

fic O

cean

CANADA

U.S.A.Vancouver

Vancouver

Island

GordaPlate

SanAndreas

fault

Cape Mendocino

Humboldt Bay

CALIFORNIA

OREGON

WASHINGTON

NorthAmerica

Plate

Puget Sound

Portland

Seattle

Salmon River estuary

Alsea BayYaquina Bay

Grays Harbor

ExplorerPlate

JuandeFucaPlate

Netarts Bay

BRITISHCOLUMBIA

Colu mbia River

Stra it of GeorgiaContinental

s lope

S eaward

edgeof

Cascadia

subducti o nzo ne

Ca

scad

iaC

hann

el

NehalemRiver

Nelson, et al., 2004

Crescent CityLagoon CreekOrick

Eel River

Locked ZoneCascadia subduction

zone

Vertical Deformation

Time-Space Relations

Mar

ine

Reco

rd

Terrestrial Record

Recurrence

Sites

Stratigraphy

Vertically Restricted Regions

Mendocino Deformation

Zone

Site Evaluation and Ranking

800

1000

1200

1400

1600

CSZ RI Means

SW Bay

Coquille Swiss HallHookton SloughGoldfinger Tim

e(y

ears

)

0

200

400

600

39.540.541.542.543.544.545.5

aCSZ

bCSZ

cCSZ

dCSZ

mean SCSZ

mean SCSZwithout SW bay

Bradley Lake

Coquille

Sixes

CrescentCity

LagoonCreek Vick

Pritchard

Eel River

Swiss Hall

South Bay

North Bay

Latitude

Recurrence Intervalsoldestage ^ oldest age ^ events events RI R. I. RI EQ RI

reference region min max included * total ++mean +/- rounded # reported $ missing %

newGoldfing 03500350350359191ZSCa8002 ,.la .te ,reGoldfing 0084084084ZSCb8002 ,.la .te ,reGoldfing 0023023023ZSCc8002 ,.la .te ,reGoldfinger, et. al., 2008 dCSZ 10010 9650 38 38 259 5 260 220 0 260Nelson, et. al., 2006 Bradley Lake 4700 4700 12 12 405 0 400 440 9 229Kelsey, et. al., 2005 Bradley Lake 4630 4460 12 12 390 8 390 390 9 223Witter, et. al., 2003 Coquille 6720 6500 12 12 578 10 580 580 19 225Kelsey, et. al., 2002 Sixes 5600 5050 11 11 508 28 510 510 14 224Carver, et. al. 1998 Crescent City 3060 2760 1, 2, 4, 5, 6 6 483 53 480 7 222Abramson, et. al., 2007 Lagoon Creek 3440 3164 2, 4, 5, 6, 7, 8 8 271 63 270 1 240Vick, et. al., 1988 North Bay 1568 1390 4, 5 5 308 37 310 2 221Pritchard, et. al., 2004 North Bay 1684 1410 2, 3, 4 4 448 45 450 4 225Valentine, 1992 North Bay 4290 4087 1, 2, 4, 6, 7, 8, 9, 10 10 379 30 380 320 7 224Carver, et. al. 1998 South Bay 1695 1542 2 2 1369 89 1370 10 228Valentine, 1992 South Bay 3366 2946 1, 2, 4, 5, 6, 7, 8 8 391 37 400 320 6 229Witter, et. al., 2002 South Bay 2289 1954 2, 3, 4 4 691 89 690 8 230Patton, et. al., 2006 South Bay 3631 3474 3, 4, 5 5 688 38 900 900 15 225Li, 1992 Eel River 1990 1739 2, 3, 4, 5 5 362 52 360 3 225

without SW bay mean 513 30 513 421

all events mean 519 41 540 494^ cal yrs BP, years before 1950.* events included in RI estimate.++ total events at site.# Recurrence Interval (RI) based on events with radiocarbon based age control.% number of earthquakes required to shorten RI to match marine RI (result in the next column over).% number of earthquakes required to shorten RI to match marine RI (result in the next column over).

Earthquake and tsunami hazard for northwestern California and southern Oregon is predominately based on estimates of recurrence for earthquakes on the Cascadia subduction zone and upper plate thrust faults, each with unique deformation and recurrence histories. Coastal northern California is uniquely located to enable us to distinguish these different sources of seismic hazard as the accretionary prism extends on land in this region. This region experiences ground deformation from rupture of upper plate thrust faults like the Little Salmon fault. Most of this region is thought to be above the locked zone of the megathrust, so is subject to vertical deformation during the earthquake cycle. Secondary evidence of earthquake history is found here in the form of marsh soils that coseismically subside and commonly are overlain by estuarine mud and rarely tsunami sand. It is not currently known what the source of the subsidence is for this region; it may be due to upper plate rupture, megathrust rupture, or a combination of the two. Given that many earlier investigations utilized bulk peat for 14C age determinations and that these early studies were largely reconnaissance work, these studies need to be reevaluated.

Recurrence Interval estimates are inconsistent when comparing terrestrial (~500 years) and marine (~220 years) data sets. This inconsistency may be due to 1) different sources of archival bias in marine and terrestrial data sets and/or 2) different sources of deformation. Factors controlling successful ar-chiving of paleoseismic data are considered as this relates to geologic setting and how that might change through time. We compile, evaluate, and rank existing paleoseismic data in order to prioritize future paleoseismic investigations. 14C ages are recalibrated and quality assessments are made for each age determination. We then evaluate geologic setting and prioritize important research locations and goals based on these existing data.

Terrestrial core transects are located in each of eight archival domains in order to evaluate archival bias and potential deformation sources for the southern Cascadia subduction zone. These domains are located in the Eel River, Humboldt Bay, Humboldt Lagoons, and Crescent City regions. In any given domain, evidence of earthquakes can be regional, local, or both. Core transects are designed to capture archival bias due to 1) interseismic de-formation in the upper plate or the megathrust, 2) rupture on upper plate thrust faults, 3) rupture on the megathrust, or 4) rupture on both. Modern bio-geochemical transects are used to calibrate paleontologic estimates.

Based on our assessment, we determine which sites need better age control, which sites need supplemental coring, and key new research areas that need to be investigated.

the missing earthquakes of humboldt county:...

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